1. Field of the Invention
The present invention relates to a microwave oven for cooking a food by supplying microwaves to the food. More specifically, the present invention relates to a microwave oven in which the impedance variation of a wave guiding tube versus the load variation of the food to be cooked is minimized, so that the output of the microwave oven would become constant regardless of the load of the food, and that the electric field distribution within the cavity would also become constant.
2. Description of the Prior Art
Generally, in a microwave oven, microwaves which have been generated in a magnetron are irradiated through a wave guiding tube into a cavity to induction-heat the food within the cavity so as to cook the food.
FIG. 1 is a schematic sectional view showing a wave guiding tube of a conventional microwave oven. FIG. 2 is an interpretation of the irradiating structure of the wave guiding tube of FIG. 1. The wave guiding tube 1 is provided with an insertion hole 9 on a side thereof for inserting a magnetron 3. On the other side of the wave guiding tube 1, there is formed a rectangular opening 7 for irradiating the microwaves from the magnetron 3 into the interior of the cavity.
The microwaves which have been generated by the magnetron 3 are irradiated through the wave guiding tube 1 into the interior of the cavity 5 as described above, so that the food within the cavity 5 would be induction-heated.
As shown in FIG. 2, it will be assumed that the power of the magnetron is P.sub.in, and the output supplied to a particular position is P.sub.out. Then P.sub.out can be calculated based on the following formulas. EQU P.sub.in =E.sub.s.sup.2 Formula 1! EQU E.sub.y =E.sub.s sin (x) Formula 2! EQU P.sub.out =(E.sub.y).sup.2 =(E.sub.s sin (x)).sup.2 =E.sub.s.sup.2 sin (x).sup.2 Formula 3!
In the Formulas 1 to 3, E.sub.s is an input electric field energy of the microwaves which are generated and guided by the wave guiding tube. E.sub.y is an output electric field energy of the microwaves, which is supplied to a particular position within the cavity 5.
The output of the magnetron 3 amounts to a square of the electric field energy E.sub.s.
Further, the microwaves which are generated by the magnetron 3 are sinusoidal waves. Therefore, the electric field energy E.sub.y which is supplied to a particular position within the cavity 5 takes the form of the electric field energy E.sub.s multiplied by sin(x). The output power P.sub.out supplied to a particular position within the cavity amounts to a square of E.sub.y.
Therefore, the output P.sub.out supplied to a particular position within the cavity 5 takes the form of P.sub.in multiplied by sin(x). The term sin(x) is varied in accordance with the load of the food to be cooked, and therefore, P.sub.out is also varied in accordance with the load of the food to be cooked.
Thus the impedance characteristics which are governed by the load of the food can be illustrated by the polar chart of FIG. 3. In this drawing, the impedance characteristics of the wave guiding tube are illustrated for the case where the frequency range of the microwaves is 2.44-2.47 GHz, and where the loads consist of 2000 cc of water, 1000 cc of water, 500 cc of water and 100 cc of water.
As shown in FIG. 3, in the case where the load is 2000 cc of water, VSWR (voltage standing wave ratio), i.e., the impedance of the wave guiding tube becomes small so as to increase the output of the microwave oven. On the other hand, in the case where the load is 100 cc of water, VSWR, i.e., the impedance of the wave guiding tube becomes large so as to decrease the output of the microwave oven.
That is, if the load of the food is large, the output of the microwave oven increases somewhat, while if the load is small, the impedance of the wave guiding tube increases so as to lower the output of the microwave oven.
Further, the impedance of the wave guiding tube is greatly varied in accordance with the variation of the load of the food, with the result that the electric field distribution within the cavity becomes non-constant.
Further, if the output of the microwave oven is to be improved, then a matching has to be realized between the impedance of the wave guiding tube and the impedance of the cavity. However, in the above described microwave oven, the wave guiding tube is designed such that it should have an impedance matching with a particular cavity. Therefore, one wave guiding tube cannot be applied to various cavities, but a separate wave guiding tube has to be designed for each cavity, this being a troublesome task.
Meanwhile, Japanese Patent Application Laid-open No. Hei-6-111933 discloses a wave guiding system for a microwave oven. In this system, the uniform heating for food within the cavity of microwave oven is improved, and the wave guiding tube has a short length. so that the positioning of components can be made easier. This is illustrated in FIG. 4. As shown in this drawing, the microwave oven includes: a pair of wave supplying holes 11a and 11b formed on a side wall; a cavity 12 for receiving a food to be cooked; and a magnetron 14 isolated from the side wall (having the wave supplying holes 11a and 11b), and disposed at a level between the wave supplying holes 11a and 11b, for generating microwaves of a frequency of .lambda..sub.g through an antenna 13. The microwave oven further includes a wave guiding tube 15 separated from the antenna 13 by a distance of .lambda..sub. g/4, covering the wave supplying holes 11a and 11b, supporting the magnetron 14 and guiding the microwaves through the wave supplying holes 11a and 11b into a cavity 12. Voltages standing waves are formed from the waves of the magnetron 14 within the wave guiding tube 15, to irradiate them through the wave supplying holes 11a and 11b into the cavity 12 so as to cook the food.
In this conventional wave guiding system, a pair of the wave supplying holes 11a and 11b are formed on a side wall of the cavity 12 at different levels. Further, the microwaves which are generated by the magnetron 14 are irradiated through the wave supplying holes 11a and 11b into the cavity 12. Therefore, only the dispersing characteristic of the microwaves is improved to uniformly heat the food. However, the variation of the output of the microwave oven cannot properly respond to the variation of the load of the food.